PHP & Case Sensitivity

FYK (updated)

Case sensitive (both user-defined and PHP defined)

  • variables
  • constants ->>check Amendment 1
  • array keys
  • class properties
  • class constants

Case insensitive (both user defined and PHP defined)

  • functions
  • class constructors
  • class methods
  • keywords and constructs (if, else, null, foreach, echo etc.)



Variables in PHP are represented by a dollar sign followed by the variable's name. The variable name is case-sensitive.

Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscores, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'


  1. Class constants are always case-sensitive. Global constants declared with const are always case-sensitive. It should be noted that this applies only to the shortname of the constant, while namespaces in PHP are always case-insensitive. Constants declared with define() are case-sensitive by default

Some useful Links

  1. Userland Naming Guide
  2. Why are functions and methods in PHP case-insensitive?
  3. Are PHP functions case sensitive?
  4. Are PHP keywords case-sensitive?
  5. Are PHP function names case-sensitive or not?
  6. Source of PHP Case Sensitive

Why is PHP partially case senstive?

I can only speculate that this stems from very early versions, probably PHP/FI 2.0. The manual explicitely states:

Keep in mind that PHP/FI function names are not case sensitive.

Most user input, such as GET and POST parameters, has always been registered as global variables, back then. Treating these as case insensitive would likely have caused issues, and supposedly therefore all variables have been treated as being case sensitive.

From what I can tell these have been the only kinds of identifiers in PHP/FI 2.0. All others have been introduced later, apparently mimicking the case-insensitive function names.

Constants, which are special, have only been introduced as of PHP 4 (the PHP 3 manual mentions "constants", but these are nowadays referred to as "literals"). For some mysterious reason (maybe no consensus could be found), it had been decided to allow constant identifiers to be define()d either case sensitive or insensitive on the developers discression. Interestingly, while define() defaults to case sensitive constants, the respective C counterparts (REGISTER_*_CONSTANT) default to case insensitive.